text: There are 2 ortho positions, 2 meta positions and 1 para position on benzene when a group is attached to it. When a group is an ortho / para director with ortho and para positions reacting with the same partial rate factor, we would expect twice as much ortho product as para product due to this statistical effect.  However, the partial rate factors at the ortho and para positions are not generally equal.  In the case of a fluorine substituent, for instance, the ortho partial rate factor is much smaller than the para, due to a stronger inductive withdrawal effect at the ortho position.  Aside from these effects, there is often also a steric effect, due to increased steric hindrance at the ortho position but not the para position, leading to a larger amount of the para product.
  The effect is illustrated for electrophilic aromatic substitutions with alkyl substituents of differing steric demand for electrophilic aromatic nitration.
  The methyl group in toluene is small and will lead the ortho product being the major product. On the other hand, the t-butyl group is very bulky (there are 3 methyl groups attached to a single carbon) and will lead the para product as the major one. Even with toluene, the product is not 2:1 but having a slightly less ortho product.